Field of the Invention
The present disclosure relates to an X-ray shield grating and an X-ray Talbot interferometer including the X-ray shield grating.
Description of the Related Art
Gratings including a structural body having a periodic structure have been used as optical elements for various apparatuses. In particular, gratings formed of metal with a high X-ray absorptivity have been used as X-ray shield gratings for non-destructive tests for objects and in the area of medical care.
As a use application of X-ray shield gratings, shield gratings in X-ray Talbot interferometers have been known. An imaging method using an X-ray Talbot interferometer (X-ray Talbot interference method) is one of methods for acquiring information of a subject by using a phase change of an X ray by the subject.
An X-ray Talbot interference method will be briefly explained below. In an X-ray Talbot interferometer, spatially coherent X rays pass through a diffraction grating which diffracts X rays and through a subject, and forms an interference pattern. At a position where the interference pattern is formed, a shield grating for periodically shielding X rays is arranged to form moire. The moire is detected by a detector, and information of the subject can be obtained from the detection result.
General X-ray shield gratings used in the X-ray Talbot interference method have a structure in which X-ray transmission portions and X-ray shielding portions are arranged periodically. The X-ray shielding portions often include a high-aspect-ratio structural body (the aspect ratio represents the ratio of height or depth h to width w (h/w) of a structural body) formed of metal with a high X-ray absorptivity, such as gold.
Furthermore, a shield grating having such a structure may be used not to form moire by partially shielding X rays which form the interference pattern as described above but to improve the spatial coherency of X rays. The shield grating used as described above is called a source grating (or a light source grating). A shield grating arranged at a position where an interference pattern is formed may be called an analysis grating. Both the source grating used to improve the coherency of X rays and the analysis grating used to form moire are X-ray shield gratings. Therefore, hereinafter, these gratings will be called a source grating and an analysis grating when it is necessary to distinguish between them, and they will be collectively called X-ray shield gratings when both the gratings are referred to. Furthermore, hereinafter, simple expressions, such as a transmission portion, a shielding portion, and a shield grating, represent an X-ray transmission portion, an X-ray shielding portion, and an X-ray shield grating, respectively.
When a source grating is arranged between an X-ray source and a diffraction grating, a state in which microfocus X-ray sources are virtually arranged can be generated. An X-ray source with a smaller focal point (X-ray generation part) has a higher spatial coherency of X rays generated from the X-ray source. Therefore, the use of such a source grating improves the spatial coherency of X rays. A Talbot interference method, which is a type of Talbot interference method implemented by generating the state in which microfocus X-ray sources are virtually arranged, may be called a Talbot-Lau interference method, in order to distinguish it from a Talbot interference method using no source grating.
Planar shield gratings are effective for the case where a Talbot interference method is used with parallel light (parallel X rays), which is used in a synchrotron radiation facility. However, in a Talbot interference method using an X-ray source which emits divergent light (divergent X rays), such as an X-ray tube used in a laboratory, a deviation between the traveling direction of X rays and the height direction of shielding portions increases as the distance from the optical axis (X-ray axis) increases, and so-called vignetting occurs. Thus, X rays which are desired to transmit through a shield grating are also shielded. Therefore, an insufficient X-ray transmission contrast is obtained, and the amount of X rays which reach a detector decreases. Accordingly, the contrast of X-ray intensity distribution detected by the detector decreases in a region distant from the optical axis.
Japanese Patent Laid-Open No. 2007-206075 discloses a method for making the traveling direction of X rays and the height direction of shielding portions the same by bending at least one of a source grating, a diffraction grating, and an analysis grating used for a Talbot interferometer.